Optical signal receiver and optical space transmission system

ABSTRACT

An optical signal receiver comprising an opto-electric converter for converting an optical signal transmitted from a remotely opposed transmitter into an electric signal, a reproduction circuit for reproducing a data signal from an output of the opto-electric converter, a fixed signal generation circuit for generating a fixed signal having a logic level fixed to 0 or 1, a switch for selectively outputting either the data signal reproduced by the reproduction circuit or the fixed signal generated by the fixed signal generation circuit, and a control circuit for detecting an abnormal state of optical signal transmission and controlling the switch. The control circuit is adapted to output the fixed signal from the switch, when it detects the abnormal state while outputting the data signal from the switch. The optical signal receiver constitutes an optical space transmission system with transmitter for transmitting an optical signal.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to an optical signal receiver for detectingoptical signals transmitted from a remotely opposed transmitter and alsoto an optical space transmission system comprising such a receiver andadapted to convey information to it.

2. Related Background Art

With a conventional optical space transmission system, a signal to betransmitted from a transmitter is modulated into an optical signal,which is then transmitted from the transmitter to a receiver by way ofthe atmosphere. Upon receiving the signal, the receiver demodulates theoptical signal transmitted from the transmitter so that the informationthat the signal represents can be conveyed from the transmitter to thereceiver by way of the atmosphere.

However, an optical space transmission system using a light beam as anoptical signal has a disadvantage that the optical signal can beaffected and attenuated by natural phenomena such as rain falls and fogsin the atmosphere. As means for coping with this problem, it is a commonpractice to detect the DC component level of the light beam received bythe receiver in order to judge if the optical signal receiver isoperating properly to receive the optical signal transmitted from theremote transmitter or not.

When abnormal communications arise in such a conventional spacetransmission system due to a natural phenomenon such as a rain fall or adense fog in the atmosphere operating as transmission paths of lightbeams, some of the optical receivers of the system may be forced to keepon outputting incorrect signals as long as the natural phenomenoncontinues. Then, as a result, the computer network devices receiving theoutput signals from such optical receivers can go down such that thedevices may have to be initialized in order to restart their operations.Then, even if the optical space communication is returned to normalduring the initialization, the device cannot resume its operation atleast during the initialization period.

Additionally, when an optical transmission system has backup routes sothat some or all of the transmission paths for optical signals in anoptical transmission system may be switched to respective wiredtransmission paths when abnormal communications arise on thosetransmission paths, the computer network devices connected to theswitched transmission paths cannot decide to switch the transmissionpath on the basis of the presence or absence of a bit stream receivedfrom the optical receiver so that they are often accompanied by aproblem of difficulty of switching the transmission path and aninterrupted telecommunication.

SUMMARY OF THE INVENTION

In view of the above identified circumstances, an object of the presentinvention is to provide an optical signal receiver and an optical spacetransmission system that can minimize the influence of any abnormaltransmission of an optical signal on the computer network devicesconnected to the system by suspending the output of the received signalthat may be incorrect so that the proper operation of the entiretransmission system can be restored quickly.

In an aspect of the invention, the above object is achieved by providingan optical signal receiver comprising:

an opto-electric converter for converting an optical signal transmittedfrom a remotely opposed transmitter into an electric signal;

a reproduction circuit for reproducing a data signal from an output ofthe opto-electric converter; a fixed signal generation circuit forgenerating a fixed signal having a logic level fixed to 0 or 1;

a switch for selectively outputting either the data signal reproducedfrom the reproduction circuit or the fixed signal generated by the fixedsignal generation circuit; and

a control circuit for detecting an abnormal state of optical signaltransmission and controlling the switch,

the control circuit being adapted to output the fixed signal from theswitch, when it detects the abnormal state while outputting the datasignal from the switch.

In another aspect of the invention, there is also provided an opticalspace transmission system comprising:

a transmitter for transmitting an optical signal; and

an optical signal receiver as defined above that is remotely opposed tothe transmitter and adapted to receive the optical signal transmittedfrom the transmitter.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic block diagram of an embodiment of optical spacetransmission system according to the invention.

FIG. 2 is a schematic block diagram of the clock extraction circuit ofFIG. 1, illustrating a possible circuit configuration thereof.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Now, the present invention will be described in greater detail byreferring to the accompanying drawings that illustrate a preferredembodiment of the invention.

FIG. 1 is a schematic block diagram of an embodiment of an optical spacetransmission system according to the invention. In FIG. 1, a transmitteris shown on the left and comprises a main signal input section 1 forreceiving a main signal to be transmitted to a remotely opposed opticalsignal receiver and an auxiliary signal generation circuit 2 forgenerating an auxiliary signal giving an optical DC component levelwhich is detected by the optical signal receiver. The main signal fromthe main signal input section 1 and the auxiliary signal generated bythe auxiliary signal generation circuit 2 are inputted to a multiplexer3 adapted to multiplex the main signal and the auxiliary signal. Theelectric signal produced as a result of the multiplexing operation ofthe multiplexer 3 is then inputted to an electro-optic converter 4 andconverted into an optical signal. The electro-optic converter 4comprises a laser drive circuit 5 and laser diode 6 that operates as alight source. Lenses 7, 8 and 9 are sequentially arranged on the lightpath of the transmitter that is found in front of the laser diode 6.

On the other hand, an optical receiver is shown on the right side inFIG. 1 and comprises lenses 11, 12 and 13 sequentially arranged on thelight path of the optical receiver for receiving the optical signal fromthe transmitter and an opto-electric converter 14 arranged on the lightpath behind the lens 13. The opto-electric converter 14 comprises alight receiving element 15 and an electric circuit 16, whose outputterminal is connected to an amplifier 17.

The output of the amplifier 17 is connected to an auxiliary signaldetection circuit 19 by way of a second filter 18 for extractingauxiliary signals and also to a main signal detection circuit 21 by wayof a first filter 20 for extracting main signals. Additionally, theoutput terminal of the amplifier 17 is connected to a clock extractioncircuit 22 and the output terminals of the clock extraction circuit 22,the auxiliary signal detection circuit 19 and the main signal detectioncircuit 21 are connected to a central processing unit (CPU) 23.

Additionally, the output terminals of the amplifier 17 and the clockextraction circuit 22 are connected to a wave shaping circuit 24 and theoutput terminal of the wave shaping circuit 24 is connected to a switch(signal output switch) 26 adapted to selectively output either theoutput of the wave shaping circuit 24 or the output of a fixed signalgeneration circuit 25 for generating a fixed signal having a logic levelfixed to 0 or 1. The output terminal of the fixed signal generationcircuit 25 is also connected to the switch 26. The switching operationof the signal output switch 26 is controlled by the CPU 23 and theoutput terminal of the signal output switch 26 is connected to a signaloutput section 27.

With the above described arrangement, the main signal inputted to themain signal input section 1 of the transmitter is multiplexed with theauxiliary signal transmitted from the auxiliary signal generationcircuit 2 by the multiplexer 3 and then the multiplexed signal isconverted into an optical signal by the electro-optic converter 4. Theoptical signal is then outputted to the remotely opposed optical signalreceiver by way of the lenses 7, 8 and 9.

The light beam carrying the optical signal transmitted from thetransmitter is then made to pass through the lenses 11, 12 and 13 and isreceived by the light receiving element 15 of the opto-electricconverter 14 of the optical signal receiver. The received optical signalis then converted into an electric signal by way of the electric circuit16 and the produced electric signal is outputted to the amplifier 17.

The electric signal amplified by the amplifier 17 is then outputted tothe wave shaping circuit 24 along with the clock signal extracted by theclock extraction circuit 22, which wave shaping circuit 24 thenreproduces the data signal on the basis of the signal it receives andoutputs the data signal to the signal output switch 26. In other words,the wave shaping circuit 24 is provided with a reproduction means forreproducing the data signal.

The electric signal amplified by the amplifier 17 is also made to passthrough the auxiliary signal extraction filter 18, which extracts theauxiliary signal component and outputs it to the auxiliary signaldetection circuit 19. The auxiliary signal detection circuit 19transforms the amplitude of the inputted auxiliary signal into avoltage, which voltage is then outputted to the CPU 23. Since theauxiliary signal generated by the auxiliary signal generation circuit 2of the transmitter is constantly held to a fixed level, the level of thereceived auxiliary signal rises or falls in proportion to the rise orfall of the level of the DC component of the received light beam.Therefore, the rise or fall of the level of the DC component of thereceived light beam can be detected by observing the rise or fall of thelevel of the received auxiliary signal.

The main signal component of the electric signal amplified by theamplifier 17 is then extracted by the main signal extracting filter 20and outputted to the main signal detection circuit 21, which main signaldetection circuit 21 transforms the amplitude of the inputted mainsignal into a voltage and outputs the voltage to the CPU 23. Since theAC component of the light beam is the data signal component, the ACcomponent of the received light beam is equivalent to the main signalthat is the data signal transmitted from the transmitter.

The electric signal amplified by the amplifier 17 is also outputted tothe clock extraction circuit 22, which clock extraction circuit 22 thenextracts the clock component of the signal and sends a voltage signal tothe CPU 23 to notify the latter if the signal is synchronized or not.

More specifically, the level of the DC component, that of the ACcomponent and the information indicating the status of synchronismprovided by the clock extraction circuit 22 are inputted to the CPU 23and the CPU 23 analyses each piece of information. If the CPU 23determines that one or more pieces of the information are found underthe predetermined respective threshold levels, it causes the signaloutput switch 26 to be switched to output a fixed signal having a logiclevel fixed to 0 or 1 to the signal output section 27.

Thereafter, the CPU keeps on extracting the level of the DC component,that of the AC component and the clock component of the data signalindicating the status of synchronism and, when it determines that allthe values are found above the predetermined respective thresholdvalues, it immediately causes the signal output switch 26 to be switchedto output the data signal from the wave shaping circuit 24 to the signaloutput section 27. Note that the threshold values to be used fordetermining if the level of the DC component and that of the ACcomponent are normal or not may be made to respectively have hysteresischaracteristics.

Thus, with this arrangement, the system is protected against outputtingan incorrect reception signal if an abnormal communication status occursin the optical space transmission system so that the computer networkdevices connected to the optical signal receiver are prevented frombeing adversely affected by the signal.

FIG. 2 is a schematic block diagram of the clock extraction circuit 22of FIG. 1, illustrating a possible circuit configuration thereof.Referring to FIG. 2, the output terminal of phase comparator 30 isconnected to a voltage control oscillator (VCO) 31 by way of point A andthe output of the VCO 31 is fed back to the phase comparator 30 by wayof a loop filter 32.

Thus, the optical signal transmitted from the remotely opposedtransmitter and received by the receiver is transformed into an electricsignal by the latter and then the electric signal and a clock signaloutputted from the VCO 31 through the loop filter 32 are compared fortheir phases so that the voltage outputted from the VCO 31 is modifiedas a function of the relative shift of the phases of the two signals. Inother words, the VCO 31 changes the frequency of the clock signalaccording to the change in the voltage outputted from the phasecomparator 30. The voltage output from the phase comparator 30 is heldto a constant level when the phase of the electric signal and that ofthe clock signal are made to agree with each other. Therefore, when thevoltage at point A is held to a constant level, the clock component ofthe data signal can be extracted in a synchronized manner.

When the level of the DC component, that of the AC component and theclock component of the data signal indicating the status of synchronismare inputted to the CPU 23 and the CPU finds that an abnormalcommunication exists in the system, the CPU 23 causes the signal outputswitch 26 to be switched so that the fixed signal generation circuit 25may output a fixed signal having a logic level fixed to 0 or 1 to thesignal output section 27.

Alternatively, it may be so arranged that the switched state ismaintained for a predetermined period of time, during which the CPU 23keeps on analyzing the status of synchronism based on the level of theDC component, that of the AC component and the clock component of thedata signal. When the CPU 23 determines that all the values are foundabove the predetermined respective threshold values and hence a state ofnormal communication is present at the end of the predetermined periodof time, it immediately causes the signal output switch 26 to beswitched to output the data signal from the wave shaping circuit 24 tothe signal output section 27.

The computer network devices that are adapted to switch the transmissionpath to the backup route when an abnormal communication arises in theoptical space transmission system are mostly so designed that theyswitch the respective transmission paths on the basis if there is a bitstream received from the optical signal receiver or not. Since theoverall throughput of the computer network system can be lowered if sucha switching operation is conducted frequently, it is normally soarranged that the operation of switching the transmission path is notrealized when the period of time during which the bit stream from theoptical signal receiver is absent is short.

In other words, even if an abnormal communication arises frequently inthe optical space transmission system, the operation of switching thetransmission path is not realized so long as such an abnormalcommunication continues for only a short period of time. Therefore, withthe above described embodiment, a time period to be used as referencefor determining that the absence of a bit stream is set long and thecomputer network devices should switch the respective transmission pathsis provided so that an abnormal communication is determined to existwhen the time period of the absence of a bit stream exceeds thereference time period. With this arrangement, the computer networkdevices reliably switch the respective transmission paths whenever anabnormal communication arises in the optical space transmission system.

As described above in detail, an optical signal receiver and an opticalspace transmission system according to the invention can minimize theinfluence of any abnormal transmission of an optical signal on thecomputer network devices connected to the system by suspending theoutput of the received signal that may be random and incorrect so thatthe proper operation of the entire transmission system can be restoredquickly.

1-12. (canceled)
 13. An optical signal receiver comprising: anopto-electric converter for converting an optical signal transmittedfrom an opposed transmitter into an electric signal; a reproductioncircuit for reproducing a data signal from an output of saidopto-electric converter; a fixed signal generation circuit forgenerating a fixed signal; and a control circuit for detecting anabnormal state of optical signal transmission, said control circuitcausing said fixed signal generation circuit to output the fixed signal,when it detects the abnormal state while the data signal is outputtedfrom said reproduction circuit.
 14. An optical signal receiver accordingto claim 13, wherein the optical signal includes a main signal and anauxiliary signal giving a DC component level and said control circuitcomprises means for detecting an AC component level of the opticalsignal, means for detecting the DC component level of the opticalsignal, means for comparing the detected AC component level and thedetected DC component level with respective predetermined thresholdvalues, and a clock extraction circuit for extracting a clock componentfrom the electric signal and detecting a synchronized or unsynchronizedstate, said control circuit being adapted to determine that an abnormalcommunication state has occurred when it detects at least a state of theAC component level falling under the threshold level, a state of the DCcomponent level falling under the threshold level and/or anunsynchronized state.
 15. An optical signal receiver according to claim14, wherein said means for detecting the AC component level of theoptical signal comprises a first filter for extracting the main signalfrom the electric signal and a main signal detection circuit fortransforming an amplitude level of the main signal extracted by thefirst filter into a voltage and said means for detecting the DCcomponent level of the optical signal comprises a second filter forextracting the auxiliary signal from the electric signal and anauxiliary signal detection circuit for transforming an amplitude levelof the auxiliary signal extracted by the second filter into a voltage.16. An optical signal receiver according to claim 14, wherein saidreproduction circuit comprises a wave shaping circuit for reproducingthe data signal on the basis of the clock component extracted by saidclock extraction circuit and the electric signal outputted from saidopto-electric converter.
 17. An optical signal receiver according toclaim 14, wherein said clock extraction circuit comprises a phasecomparator, a voltage control oscillator adapted to receive an output ofthe phase comparator and a feedback loop for feeding back an output ofthe voltage control oscillator to the phase comparator by way of a loopfilter.
 18. An optical signal receiver according to claim 14, furthercomprising a fixed signal generation circuit for generating a fixedsignal, wherein said control circuit causes said fixed signal generationcircuit to output the fixed signal, when it detects the abnormal statewhile the data signal is outputted from said reproduction circuit. 19.An optical signal receiver according to claim 18, wherein said controlcircuit causes said reproduction circuit to output the data signal whenthe AC component level and the DC component level exceed the respectivepredetermined threshold values and said clock extraction circuit detectsthe synchronized state while said fixed signal generating circuit isoutputting the fixed signal.
 20. An optical signal receiver according toclaim 18, wherein said control circuit causes said reproduction circuitto output the data signal when the AC component level and the DCcomponent level exceed the respective predetermined threshold values andsaid clock extraction circuit detects the synchronized state at the endof a predetermined period of time during which said fixed signalgenerating circuit keeps on outputting the fixed signal.
 21. An opticalspace transmission system comprising: a transmitter for transmitting anoptical signal; and an optical signal receiver according to any ofclaims 13 through 20, said optical signal receiver being opposed to saidtransmitter and adapted to receive the optical signal transmitted fromsaid transmitter.
 22. An optical signal receiver according to claim 13,wherein said fixed signal has a logic level fixed to 0 or 1.